1. Field of the Invention
The present invention relates to an image forming apparatus, a process cartridge, and an image bearing member. The image forming apparatus, the process cartridge, and the image bearing member are applied to a photocopier, a facsimile machine, a laser printer, a direct digital platenmaker, etc.
2. Discussion of the Background
In recent years, image forming apparatuses such as laser printers, digital photocopiers, etc., that employ an electrophotographic system have been improved with regard to image quality and stability thereof and have now become widely diffused. Such image forming apparatuses use an image bearing member such as a photoreceptor or photoconductor the surface of which is charged and irradiated to form a latent electrostatic image thereon, followed by development to obtain a visible image.
Widely used photoconductors are mainly formed of organic materials because these materials are advantageous in terms of cost, productivity, variety of choices of materials used, impact on global environment, etc. Such organic photoconductors are formed of a photosensitive layer mainly containing a photosensitive material, and can be classified into two main types: A single-layer type in which a single layer assumes both a charge generation function and a charge transport function, and a laminate type having a charge generation layer assuming a charge generation function and a charge transport layer assuming a charge transport function.
The mechanism of forming latent electrostatic images on an image bearing member of a functionally separated laminate type involves irradiating a uniformly charged image bearing member with light, which then passes through the charge transport layer and is absorbed in charge generation material in the charge generation layer, resulting in generation of charge (actually a pair of charges). One in the pair is infused into the charge transport layer at the interface between the charge generation layer and the charge transport layer and moved through the charge transport layer by an electric field to the surface of the image bearing member where it then neutralizes the surface charge provided by charging, resulting in formation of a latent electrostatic image. Organic photoconductors having such a laminate structure are advantageous in terms of stability of electrostatic characteristics and durability, so that organic photoconductors are currently the main type of image bearing members in use today.
With improvements in the development material and the image forming apparatus itself in addition to the image bearing member, the quality of images produced by an image forming apparatus using an organic photoconductor has been markedly improved, which leads to expansion of application scope of the image forming apparatus. In recent years, as a result of full colorization and high-speed performance, application of such an image forming apparatus to other fields, the printing field in particular, has been strongly demanded. In the printing field, since printing is repetitively performed at a high speed for an extended period of time, the image bearing member is required to have improved abrasion resistance and durability to meet the demand for long life as well as high speed.
There are a great many technologies with regard to a protection layer that is provided to improve abrasion resistance and durability of the image bearing member.
For example, unexamined published Japanese patent application publication no. (hereinafter referred to as JP-) S56-48637-A describes a technology of using a curable resin for a charge transport layer, JP-2004-302452-A, a technology of using a cured cross-linked layer as a surface layer, and JP-2005-99688-A, a technology of using filler particulates in a cured surface layer. However, although the abrasion resistance and durability of an image bearing member has been improved, the cleaning blade tends to turn inward or outward, chip off, or be abraded soon, which inevitably causes bad cleaning performance or filming, etc. resulting in deterioration of the image quality.
In addition, toner remaining on an image bearing member resulting from poor cleaning performance easily attaches to a charging roller, thereby accelerating the image quality deterioration. Therefore, in addition to improvement of the abrasion resistance and durability, it is necessary to stabilize the cleaning ability by preventing the cleaning blade from turning inward or outward, chipping, abrading, etc.
A known technology to improve the cleaning ability is to roughen the surface of an image bearing member.
For example, JP-S60-9259-A describes a technology in which a surface roughening agent is dispersed in a photosensitive layer, JP-S53-92133-A describes a technology of controlling a drying speed of a protection layer, JP-H02-139566-A describes a technology of using a film-like polishing agent for mechanical roughening, and JP-H02-150850-A describes a technology of mechanically roughening the surface of an image bearing member by using spherical shot or abrasive particles having a spherical form and sandblasting the surface of an image bearing member. However, although successful in improving the cleaning ability by surface roughening, it tends to increase abrasion of the image bearing member and/or cause damage during repetitive use, which causes sudden, drastic deterioration in the cleaning ability.
Another technology described as the technology that improves the cleaning ability is one in which a lubricant material is applied to the surface of an image bearing member.
For example, JP-2002-287567-A describes a technology in which a lubricant is applied to an image bearing member to give it a friction index of from 0.1 to 0.4, and JP-2007-79244-A describes a technology in which a lubricant is applied to an image bearing member having a cross-Linked layer having a surface roughness Rz of from 0.4 to 1.0 (μm). These technologies contribute to keeping the friction index of the surface of an image bearing member low and thus are good to improve the cleaning ability, and stably sustain its effect. Furthermore, as described in JP-2005-249901-A, the lubricant material applied to the surface of an image bearing member also functions as a protection material for the image bearing member so that the lubricant material improves the abrasion resistance and durability of the image bearing member as well as to reduce the friction index thereof.
However, although a lubricant applied to the surface of an image bearing member contributes to stabilization of the behavior of a cleaning blade, the edge portion of the blade is constantly in contact with the image bearing member, which may accelerate abrasion of the blade edge. In addition, the amount of lubricant material that remains on the surface of an image bearing member decreases when images having a large image area are repetitively printed, which may cause bad cleaning performance and/or filming. Therefore, although these technologies improve both the abrasion resistance and durability of an image bearing member without degrading the cleaning ability, such effects are not sufficiently sustained. Depending on the field of application, for example, in particular the printing field, images having a large image area may be repetitively printed in full color over an extended period of time. To handle such a demanding operation, it is preferable that the image bearing member be free from abrasion or damage and the initial surface status thereof stably sustained. Also, the amount of lubricant material applied and the behavior of a cleaning blade need to be stabilized.
Furthermore, as another technology to provide a lubricant to the surface of an image bearing member, a lubricant is mixed or kneaded with and dispersed in or externally added to a toner. JP-H11-327303-A and JP-2008-139804-A describe a technology of providing such a toner to the image bearing member to apply the lubricant to the surface thereof.
This approach is good for cost reduction of an image forming apparatus. On the other hand, the application of the lubricant tends to be inferior, thereby causing the attachment amount of the lubricant to be insufficient or uneven.
As a result, the image stability deteriorates and the lubricant applied to the surface of an image bearing member is not sufficiently effective.
Thus, due to improvements in abrasion resistance and durability, the image bearing member now has a longer working life. However, this improvement is not good in light of the cleaning blade because the cleaning blade is turned inward or outward, chipped, or abraded quickly, which causes deterioration of cleaning performance, filming, contamination of the charging roller, etc., resulting in deterioration of the image quality. That is, if improvement of the length of the working life of an image bearing member causes bad cleaning performance, filming, etc., which results in deterioration of the image quality, such improvement does not contribute to the elongation of the working life in a true sense.
One of the reasons why deterioration of the cleaning ability or occurrence of filming is caused by improving the abrasion resistance or durability of an image bearing member is that the lubrication property of the surface thereof deteriorates because the lubricant is hardly refreshed (replenished).
When the lubricant property of the surface worsens, friction between the cleaning blade and the surface increases, thereby pulling in the blade and turning it outward. If this state continues, the blade is easily chipped or cracked, which leads to bad cleaning performance or filming. Therefore, since the way a lubricant is applied to the surface of an image bearing member determines how well the surface is lubricated, it should also be effective to improve the cleaning ability and prevent filming. However, although the cleaning ability is improved by application of a lubricant and the operation of the blade is stabilized, the edge of the cleaning blade is constantly in close contact with the surface of an image bearing member. Therefore, abrasion easily starts from the edge portion of the blade, resulting in deterioration of sustainability of improvement in the cleaning ability and prevention of filming.